<style>
  body { overflow-y: scroll }
  div {
    width: 10px;
    height: 10px;
    background-color: #d64203;
  }
</style>

<body>
  <div id="a"></div>
  <div style="height: 50px"></div> <!-- Unaffected by scrollbars. -->
  <div id="c"></div>
  <div style="height: 50px"></div> <!-- Unaffected by scrollbars. -->

  <!-- Deliberately make scrollbars appear. -->
  <div style="width: 500px; height: 500px; background-color: black">
</body>

<script>
  // client{Width, Height} consist of the area _inside_ the scrollbars, so these
  // manually calculated units reflect the fact that with 'overflow: scroll' set,
  // viewport units are sized taking the scrollbars into account. Since we're
  // only dealing with 'overflow-y' here, only the width units are affected.
  var vw = 0.01 * document.body.clientWidth;
  document.getElementById('a').style.width = (50 * vw) + "px";
  document.getElementById('c').style.width = (35 * vw) + "px";
</script>
